What happened to Cory Richards on Mount Everest on April 28 when the climber became ill, and how that will affect the rest of the National Geographic Society’s 2012 expedition to Everest is not yet clear.

Richards had climbed to 23,000 feet with Conrad Anker, the expedition’s leader, on a reconnaissance and acclimatization hike toward the West Ridge route they planned to use to ascend the peak. Various accounts report that they turned back because Richards experienced shortness of breath and chest pain, or that his symptoms began after they turned back because of dangers posed by rock fall. But the pain in his left side led to a speedy descent of more than 5,000 feet, some of it with Richards being hauled in a sked, before he was finally airlifted from Base Camp seven hours later.

Cloud cover prevented a helicopter rescue from Camp 1, and the team decided that although crossing the Khumbu Icefall in the afternoon was risky, it was a better choice than keeping Richards at an elevation of 19,900 feet overnight.

“Late afternoon is not the time to pass through the Khumbu, which was steaming and creaking ominously, but we had no choice,” National Geographic writer Mark Jenkins reported in the Field Test blog. “It was a matter of weighing risks. Despite his condition, Cory flew down through the Icefall.”

From Base Camp, elevation 17,600 feet, Richards was airlifted by helicopter, despite adverse weather conditions, to Lukla and then on to a hospital in Kathmandu.

A pulmonary embolism, a blockage in the main arteries of the lungs often caused by blood clots, was suspected as the diagnosis, but the tests have come back negative.

The results of the tests have “turned up nothing,” says Dr. Peter Hackett, executive director of the Institute for High Altitude Medicine in Telluride who spoke with Richards’ doctor in Kathmandu earlier this week.

“Nobody knows what it was, but it went away,” Hackett says. “They definitely ruled out a pulmonary embolism with a CAT scan, and Cory’s heart seemed OK. They think he’s OK to go back up, but the National Geo team is a pretty nervous about it.”

There are other possible causes for the pain on the left side of his chest, Hackett says, but it’s all speculation. As is what would happen to Richards were he to return to altitude.

“There’s no diagnosis, so there’s no way to know if it might have had to do with altitude or not,” Hackett says. “The big question is can he go back up without knowing exactly what it was?” Without knowing, it’s possible he could go back to altitude and have the same problem again.

“Stranger things have happened, but the good news is that he’s fine. There’s no major problem,” Hackett says.

National Geographic has reported that Richards will return to Everest, though the decision on whether he will attempt the West Ridge is likely to fall to the expedition leader, Anker.

In a May 1 dispatch, Anker said, “Regarding Cory, it’s wait and see. And something like this, where a climber’s health is compromised at certain elevations, still 2,000 meters below the summit — it’s not the prudent thing to have the climber come back up and go back into the invisible grip of altitude.”

Anker estimated then that, based on weather and snow conditions, a summit push was likely 10 or more days out.

This was Richards’ first attempt on Mount Everest (see “Getting a grip” in the March 15 issue of Boulder Weekly). He has called the expedition “the culmination of my career as a photographer and climber” in the National Geographic Field Test blog and said it “is the realization of a partnership, friendship, and mentorship that reaches beyond words.”

For additional information, photos and updates on the National Geographic expedition to Mount Everest in 2012, visit natgeo.com/ everest.